Web Services

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Web Services
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Data on the Web

• There is a huge volume of data on the web
• These data are displayed in a browser after
  transferring via http
• But displaying data is not all we want to do!

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Consider

• I have developed a sophisticated weather
  prediction model and have programmed an
  application to implement it.
• For input, the model requires ongoing and up to
  date data on temperature, wind, humidity, rain,
  etc, from various places around NZ
• This information is available on the web at
  www.metservice.co.nz
• How do I get these data into my application?

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Accessing data on the web

• Consider these alternatives
• 1. Load the information into a web browser, then
  copy or cut n paste.
• 2. Use a screen scraper program/script
• 3. Fetch the data directly using a web service

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Web services

• A web service is an online application that is
  accessed using XML technology
• Example a service to give information on the
  weather in a particular centre
• One could expose, over the web, methods including
• getTemperature(placeName)
• getHumidity(placeName)
• getWindSpeed(placeName)
• getPressure(placeName)
• A web service is not the same thing as a web
  site, ie a set of web pages served to a browser
  client.

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Web services

• NetBeans provides tools to
• Create web services
• Expose web services
• Consume web services
• A listing of web services can be found at
• www.xmethods.com

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Remote procedure calls

• Web services allow remote procedure calling over
  the internet
• But this is nothing new
• RPC can be achieved by low-level programming, eg
• TCPP/IP sockets
• Java RMI
• We want to go beyond just RPC

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What do we want

• If a web service has been created, we want it to
  be discoverable by clients on the Internet
• We want clients to be able to work out for
  themselves how to interact with the web service
• We want to work across the top application layer
  of the internet protocol suite (ie HTTP, FTP).

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Overview
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Issues to resolve

• Clients need to be able to locate the service?
• UDDI
• Clients need to know how to interact with the
  service?
• WSDL
• How will the client perform the actual
  interaction?
• SOAP
• UDDI, WSDL, SOAP are XML grammars

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Locating the service

• When you have created a service, it can be
  registered with a repository
• This can be done using UDDI (Universal
  Description Discovery and Integration)
• An XML technology for the registration of
  services
• External UDDI repositories are available from
  various vendors
• uddi.microsoft.com
• uddi.ibm.com

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Performing a look-up

• The client can lookup the service based on
  criteria like
• Services offered by a particular business
• Services in a category, eg weather services
• A specific service, eg a weather service
  offered by a given company/institute

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WSDL

• The UDDI registry will provide the client with a
  WSDL (Web Services Description Language) file
  location
• WSDL is an XML grammar that describes the
  interactions with the service
• The URL that the service is accessed through
• What methods are available?
• What XML messages does the service expect?

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Interacting with a service

• Once the client knows how to interact with the
  service, client and service pass XML messages
• These messages use the SOAP (Simple Object Access
  Protocol) grammar

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Main points to remember

• We want to do more than merely display data from
  the web
• What is a web service?
• The role of these technologies
• UDDI
• WSDL
• SOAP